It is generally known to reinforce concrete with metal elements, such as steel fibers, for example to give the matrix the required tensile properties.
As bare steel fibers may suffer from corrosion, galvanized steel fibers have been proposed to give the fibers a long term corrosion resistance. Galvanized reinforcing steel elements are especially useful for the reinforcement of concrete for construction purposes whereby the reinforced concrete will be exposed to the weather before construction begins, as for example in prefabrication construction.
However, the use of galvanized steel fibers in concrete is creating problems: during hardening of the concrete, the galvanized surface of the steel elements will react with the alkaline concrete to form zinc salts accompanied by hydrogen gas evolution.
The hydrogen gas evolution is leading to aesthetical problems as well as to strength and durability problems.
Due to hydrogen gas evolution at the interface of the metal elements and the concrete, the bond strength between the metal elements and the concrete is reduced. This is resulting in a reduction of the strength of the reinforced concrete.
The durability problem is the result of the reduction in the thickness of the zinc or zinc alloy coating due to the reaction of the zinc or zinc alloy coating in the alkaline environment.
The problems of galvanized steel fibers in concrete are described in “Effect of chemical-physical interaction between galvanized steel fibres and concrete”, T. Belleze, R. Fratesi, C. Failla, 6th RILEM Symposium on Fibre-Reinforced Concretes (FRC) BEFIB 2004, 20-22 Sep. 2004, 239-248.
To prevent the hydrogen gas evolution, the zinc surfaces can be passivated. This can be realised by treating the galvanized steel elements with a chromium based compound. Also the chromate naturally present in the concrete can be sufficient to protect the galvanized steel elements.
However, in recent years it has been recognized that hexavalent chromium raises serious environmental and health problems. Consequently, strict restrictions have been placed on the quantity of hexavalent chromium used in a number of industrial processes and products as for example cement and concrete.
Other attempts to protect the galvanised steel comprise the application of an epoxy coating on the galvanised steel. The use galvanised steel coated with an epoxy coating to reinforce concrete is for example described in JP 53-078625.
The epoxy coating acts solely as a barrier against a corrosive environment. If there are defects in the epoxy coating through which aggressive agents can penetrate the barrier, corrosion will concentrate on these areas. Defects in the coating will thus cause local hydrogen gas evolution and will result in a loss of bond strength.
Integrity of the epoxy coating is therefore essential as the film must be free from pores, cracks and damaged areas.
Epoxy coatings are fragile. Epoxy coated metal elements must therefore be handled with a lot of care during storing, transport and handling.
As the mixing of the reinforcement elements in the concrete is a robust operation whereby local damages on the surface of the reinforcement elements are unavoidable, the use of epoxy coated metal elements for the reinforcement of concrete is not a good option.
Many corrosion inhibitors known in the art such as phosphates, silicates, silanes, carbonates and carbonic acids, sulfides and mercaptoderivates, amines and sulfonates have been tested. However, these inhibitors did not give an adequate result as they were not able to avoid hydrogen gas evolution.
Therefore, obtaining an adequate protection of zinc or zinc coated metal elements without using chromium compounds and not requiring a 100% closed barrier coating remains to be a problem and efficient solutions are still needed.